New Fossil Evidence Suggests There Have Been 6 Mass Extinctions, Not 5

It has been argued that we are on the brink of the next great extinction, which would be the sixth mass extinction event that our planet has seen so far. Or at least, so we thought. According to new research, another hotly debated mass extinction should be added to the recognized list of mass extinction events. Newly gathered data indicates that a previously observed mass die-off was far more widespread than once thought. The study has been published in the Geological Society of America Bulletin.

Although there have been five traditionally recognized mass extinctions, scientists have contemplated the existence of a sixth, occurring in the Middle Permian (262 million years ago), for more than 20 years. This so called “Capitanian extinction” was proposed after scientists discovered fossil evidence for mass die-offs in rock formations in China. However, this event has remained controversial since it was only known from data gathered in tropical latitudes, with scarce evidence from higher latitudes. This led scientists to argue that it may have only been a localized event, or perhaps the start of a trend towards the Permian extinction, which took place ten million years later.

With the hope of offering some clarity on this issue, scientists from the Universities of Hull and Leeds scrutinized marine fossil ranges in the Kapp Starostin Formation of Spitsbergen, the largest island of the Norwegian archipelago Svalbard. This formation was created in cool, shelf seas of the Boreal Ocean from the skeletons of dead sponges. The researchers were looking at fossils left by brachiopods—invertebrate marine animals that started appearing at the beginning of the Cambrian period, around 570 million years ago.

Whilst making their way through the rock record, they found that brachiopod fossils abruptly disappeared for a stretch of time. “They all drop out,” co-author Paul Wignall told Science. “It’s like a blackout zone and there’s nothing around.” Slightly later on down the line, a few species seemed to recover, followed by an explosion of mollusks around 8 million years before the next mass event, the Permian extinction, which wiped out around 97% of known species.

According to the researchers, this indicates that there were indeed two severe brachiopod extinctions in the Middle to Late Permian, which were separated by a recovery phase. Furthermore, dating techniques used by the team indicate that the first occurred in the Capitanian, which would mean that this proposed sixth extinction event was indeed widespread.

“It’s the first time we can say this is a true global extinction,” lead author David Bond told Science. According to Bond, it would have been similar in terms of magnitude to the famous end-Cretaceous mass extinction that wiped out the dinosaurs.

So what caused this apparent sixth mass extinction? The researchers think that anoxia, or low levels of oxygen, may have played a part given the fact that this event seemed to coincide with an intensification of oxygen depletion. Additionally, an apparent widespread lack of carbonates across higher latitudes could also indicate a role for ocean acidification.